A problem that is common to this kind of measuring devices such as the dissolved oxygen analyzer is consumption of an electrolytic solution. The electrolytic solution into which a pair of the electrodes (an anode and a cathode) are immersed is consumed because of an electrochemical reaction on the electrodes at a time of the measurement and eventually the measurement becomes impossible. As a result of this, it becomes necessary to exchange the electrolytic solution according to the use of the measuring device. If the measuring device is of a type wherein the electrolytic solution is unexchangeable, it is necessary to exchange the measuring device itself.
The patent document 1 describes, for example, a means to restrain consumption of the electrolyte solution as a measure to reduce a frequency of exchanging the electrolyte solution in a field of the dissolved oxygen analyzer. Concretely, a cathode and an anode are connected through a wiring cable at a time when the measuring device is used, and a resistor is arranged between the cathode and the anode at a time when the measuring device is not used. With this arrangement, the electric current due to the electrochemical reaction at a time when the measuring device is not used is reduced and the consumption of the electrolytic solution can be reduced.
Meanwhile, in accordance with the dissolved oxygen analyzer having the above-mentioned arrangement, in case that the oxygen concentration in the measurement sample is low, the oxygen that originally exists in the electrolytic solution reacts on the cathode and a surplus current flows at a time of an initial measurement. As a result of this, it is not possible to secure the measurement accuracy until the existing oxygen is completely reduced so that there is a problem that the responsiveness is aggravated.
Then, in order to improve the responsiveness in case that an oxygen concentration is low, a noise removing electrode as being a guard electrode is arranged to surround the cathode, and all of the oxygen contained in the electrolytic solution outside of the guard electrode is reduced on the guard electrode so that the oxygen that is not from the measuring sample does not reach the cathode.
However, with this arrangement, in case that the oxygen concentration is high, the reaction amount of the oxygen in the measuring sample reduced on the guard electrode increases so that there is a problem that the electrolytic solution is quickly consumed. Especially, in relation to an arrangement that the guard electrode surrounds the cathode, a contact area (a reacting area) of the guard electrode to the electrolytic solution becomes bigger than the contact area of the cathode. Then, when the reduced reaction occurs, the amount of the reduced oxygen on the guard electrode becomes more than the amount of the reduced oxygen on the cathode. As a result of this, the electrolytic solution is heavily consumed.
This problem is common to not only the dissolved oxygen analyzer but also a measuring device that reduces the object substance such as oxygen that enters from the measuring sample through a permeable member by the use of the cathode that is immersed in the electrolytic solution or that oxidizes the object substance by the use of the anode that is immersed in the electrolytic solution, and that measures a concentration of the object substance in the measuring sample based on an value of an electric current that flows between the anode and the cathode at a time of reduction or oxidization.